The present invention relates to extrusion dies for the extrusion of honeycomb structures from plasticized powder batches. More particularly the invention relates to improved methods for making such extrusion dies at lower cost and in higher quality, and to the resulting dies and their use in honeycomb extrusion.
Ceramic and metallic honeycomb structures for applications such as catalyst substrates, honeycomb heaters, and the like are presently manufactured in large quantities by extrusion methods. The extrusion dies used in these processes are typically configured as flat plates comprising an inlet face for introducing a plasticized batch material to be extruded and a discharge face from which the extrudate is discharged in the form of a honeycomb.
The discharge face of a typical honeycomb extrusion die incorporates an array of shallow criss-crossing discharge slots from which the batch material emerges as an interconnected wall structure forming the cell walls of the honeycomb. The inlet face comprises an array of feedholes extending from the face inwardly toward and connecting with the bases of at least some of the discharge slots. The feedholes provide the means for supplying the slots with the batch material to be extruded. U.S. Patent No. 3,790,654 to Bagley describes honeycomb extrusion dies of this type and their use for the extrusion of honeycombs; reference may be made to that patent for a further description of these processes.
For the high volume extrusion of thin-walled ceramic honeycombs, such as the honeycombs used for the support of automotive exhaust antipollution catalysts, extrusion dies fabricated of stainless steel are presently employed. To form the fine discharge slot arrays required for the production of these thin-walled ceramics, traveling wire electrical discharge machining (wire EDM) is the process which is presently preferred.
The wire EDM process is a metal removal technique which uses electricity to erode material from a work piece. Rapid DC electrical pulses between a negative traveling wire electrode and a positively biased workpiece, through a deionized (DI) water dielectric flushing medium, precisely melt the steel material from the workpiece. This material resolidifies in and is flushed away by a pressurized jet of the DI water flushing medium.
The flushing away of the removed material is critical to EDM process stability and the accuracy of the cut slot. The wire electrode used in wire EDM processing is typically made of brass, copper or molybdenum. As this electrode is advanced into the work piece, by means of a servo system or other apparatus, the control system for the apparatus maintains a pre-set gap between the electrode and workpiece so that a very precise slot can be formed. The presence of debris in this slot can seriously interfere with slot precision, and can also result in wire breakage which further degrades the smoothness and/or uniform width of the slot.
One difficulty which has been encountered in the fabrication of extrusion dies by wire EDM machining is excessive levels of wire breakage during the slotting of these dies. The incidence of wire breakage observed has been significantly greater than experience with slot cutting in other wire EDM applications would indicate.
It therefore is a principal object of the present invention to provide an improved die fabrication method which provides more stable EDM processing and thus extrusion dies of improved slot quality and precision.
It is a further object of the invention to provide improved extrusion dies and a method for their use.
Other objects and advantages of the invention will become apparent from the following description thereof.